Attention has been focused on a technique for forming a transistor using a semiconductor thin film formed over a substrate having an insulating surface. Such a transistor is applied to a wide range of electronic devices such as an integrated circuit (IC) or an image display device (display device). A silicon-based semiconductor material is widely known as a material for a semiconductor thin film applicable to a transistor. As another material, an oxide semiconductor has been attracting attention.
For example, a transistor whose active layer includes an amorphous oxide containing indium (In), gallium (Ga), and zinc (Zn) and having an electron carrier concentration of less than 1018/cm3 is disclosed (see Patent Document 1).
Further, a transistor using an oxide semiconductor operates faster than a transistor using amorphous silicon. The transistor using an oxide semiconductor can be included in a driver circuit, a high-speed memory circuit, and the like of a display device.